1. Field of the Invention
The present invention relates to methods of operating and control systems for air conditioning systems and, more particularly, to a method of operating and a control system for control devices in multiple vapor compression refrigeration systems (chillers) whereby chillers can be stopped at a predetermined load in order that the remaining building load can be picked up by the remaining running chillers without exceeding set load capacities of the running chillers.
2. Description of Related Art
Generally, large commercial air conditioning systems include a chiller which consists of an evaporator, a compressor, and a condenser. Usually, a heat transfer fluid is circulated through tubing in the evaporator thereby forming a heat transfer coil in the evaporator to transfer heat from the heat transfer fluid flowing through the tubing to refrigerant in the evaporator. The heat transfer fluid chilled in the tubing in the evaporator is normally water or glycol, which is circulated to a remote location to satisfy a cooling load. The refrigerant in the evaporator evaporates as it absorbs heat from the heat transfer fluid flowing through the tubing in the evaporator, and the compressor operates to extract this refrigerant vapor from the evaporator, to compress this refrigerant vapor, and to discharge the compressed vapor to the condenser. In the condenser, the refrigerant vapor is condensed and delivered back to the evaporator where the refrigeration cycle begins again.
To maximize the operating efficiency of a chiller plant, it is desirable to match the amount of work done by the compressor to the work: needed to satisfy the cooling load placed on the air conditioning system. Commonly, this is done by capacity control means which adjust the amount of refrigerant vapor flowing through the compressor. The capacity control means may be a device for adjusting refrigerant flow in response to the temperature of the chilled heat transfer fluid leaving the coil in the evaporator. When the evaporator chilled heat transfer fluid temperature decreases, indicating a reduction in refrigeration load on the refrigeration system, a throttling device, e.g. guide vanes, closes, thus decreasing the amount of refrigerant vapor flowing through the compressor drive motor. This decreases the amount of work that must be done by the compressor thereby decreasing the amount of power draw (KW) on the compressor. At the same time, this has the effect of increasing the temperature of the chilled heat transfer fluid leaving the evaporator. In this manner, the compressor operates to maintain the temperature of the chilled heat transfer fluid leaving the evaporator at, or within a certain range of, a setpoint temperature.
Large commercial air conditioning systems, however, typically comprise a plurality of chillers, with one designated as the "Lead" chiller (i.e. the chiller that is started first) and the other chillers designated as "Lag" chillers. The designation of the chillers changes periodically depending on such things as run time, starts, etc. The total chiller plant is sized to supply maximum design load. For less than design loads, the choice of the proper number of chillers to meet the load condition has a significant impact on total plant efficiency and reliability of the individual chillers. In order to maximize plant efficiency and reliability it is necessary to stop selected chillers under low load conditions, and insure that all remaining chillers have a balanced load. The relative electrical energy input to the compressor motors (% KW) necessary to produce a desired amount of cooling is one means of determining the loading and balancing of a plurality of running compressors. In the prior art, however, when the building load decreased and the chillers changed capacity to follow the building load, a selected chiller was manually stopped by an operator when the total load estimated by the operator on the system dropped below the total estimated capacity of the running chillers by an amount equal to the estimated capacity of the chiller to be stopped. However, subsequent slight increases in building load required the previously stopped chiller to be started again. This stopping and starting chillers has a very detrimental effect on the efficiency and reliability of the chillers. Thus, there exists a need for a method and apparatus which determines when a chiller can be stopped so that the remaining chillers can pick up the remaining building load and which minimizes the disadvantages of the prior control methods.